The present invention relates to a image data transfer method, i.e., coding, transmitting and decoding method of moving image digital data which method is efficient, cost effective and capable of transferring rapidly moving images clearly without a loss of recording time and image quality.
Conventional transfer methods of moving image data to be used with image coding transmission systems are disclosed in JP-A-No. 62-164391 and IEEE, Trans. on Communications, Vol. COM-32, No. 3, March 1984, pp. 225 to 232.
FIG. 2 is a block diagram showing an image data transfer system according to the above conventional techniques. In the figure, reference numeral 20 designates an image data transfer unit (moving image coding unit), 22 an input video signal, 23 an A/D converter, 24 image data, 25 a frame memory whose capacity allows storage of image data of at least one frame, 26 an image coding circuit, 27 coded data, 28 a buffer memory, 29 a coded data amount control circuit, and 30 a coded data amount control signal. Reference numeral 31 represents a transmission medium, 21 an image data reproduction unit (moving image decoding unit), 32 coded data, 33 a buffer memory, 34 an image decoding circuit, 35 image data, 36 a frame memory, 37 a D/A converter, and 38 an output video signal. FIG. 3 is a graph showing an example of a change with time of the amount of coded data (hereinafter called coded data amount) generated by the image coding circuit 26 shown in FIG. 2. Generally, the coded data amount becomes small as the frame correlation becomes large, and large as the latter becomes small.
In operation of the system shown in FIG. 2, the input video signal 22 is transformed into digital image data 24 by the A/D converter 23, one frame after another every 1/30 second, and is stored in the frame memory 25. The image data 24 stored in the frame memory 25 is subjected to high efficiency coding by the image coding circuit 26 to be stored in the buffer memory 28. The coded data 27 stored in the buffer memory 28 is read out therefrom at a speed of, e.g., 1M bits per second (hereinafter represented by 1 Mbps) and is outputted to the transmission medium 31. If the content of an image in the input video signal 22 changes slowly, the coded data amount 40 generated by the image coding circuit 26 is small, such as in the case of frames a, b, c and d shown in FIG. 3. Namely, the coded data amount, 40 is smaller than the data transfer capacity 41 or level A per one frame determined by the data transmission speed of the transmission medium 31 (in this case, the data transfer capacity per frame is 1 Mbps.times.1/30 second which is approximately 32K bits). In such a case, the coded data with dummy data inserted therein is sometimes used for synchronous data transmission between the image data transfer unit 20 and the image data reproduction unit 21. On the other hand, if the content of the image in the input video signal 22 changes rapidly, or the scene represented by the input video signal 22 changes, the coded data amount 40 generated by the image coding circuit 26 may exceed the data transfer capacity 41 or level A per one frame, such as in the case of the e and f frames shown in FIG. 3. In such a case, in view of an increase of data amount in the buffer memory 28, a frame such as g frame is deleted by means of the coded data amount control circuit 29, or coding parameters are changed to use a higher data compression rate, such as in the case of the h and i frames. The coded data amount 40 is thus forcibly made small.
A constant data transmission speed has been used for the transmission medium in the prior art techniques, so that frame deletion and/or a coding parameter change has been used for rapidly moving images in order to forcibly reduce the coded data amount. Therefore, the image quality of moving images with rapid change deteriorates extremely, posing a problem associated with information transfer and image reproduction.
In order to solve the above problem, the data transmission speed of the transmission medium can be increased. However, according to the conventional image data transfer method, the data transfer speed is fixed at a constant value, so that, if a communication line is used as the transmission medium, an excessive rise in communication cost occurs. Also, if a package-based medium such as CD-ROM (Compact Disc Read Only Memory) is used as the transmission medium, the total image recording time is considerably reduced. Each of them poses another issue to be solved.
It has been desired therefore to realize an image data transfer method which can solve the above problems and allow rapidly moving images to have the same quality as that of gently moving images, with less reduction of total image recording time, and with cost effectiveness.